An Evolutionary Model for Space Solar Power
Nicholas Boechler, Sameer Hameer, Sam Wanis, Narayanan Komerath
School of Aerospace Engineering, Georgia Institute of Technology, Atlanta GA 30332.
[email protected] 404-894-3017
Abstract. The primary difficulties with the dream of Space Solar Power (SSP) for earth, are the extreme launch costs of solar
power satellites to Geosynchronous Earth Orbit (GEO), and the absence of an evolutionary path to SSP. This makes the cost-tofirst-power unacceptably high. We present a 3-stage approach to SSP, and lay out the problems and opportunities. The key idea is
to use space assets initially for global transmission and distribution, rather than generation, establish the infrastructure, and then
add space-based power generation to a revenue-generating power grid. In the first stage, a Space Power Grid is established with
satellites 1,200 km above Earth, distributing earth-generated beamed microwaves to other satellites and ground receivers. This
boosts the earth-based alternative power industry (wind and solar-thermal) by providing transmission between any points on
earth, and accommodating fluctuations in demand and supply. It also satisfies strategic needs for emergency power delivery. In
Stage 2, direct power conversion technology will augment the existing space power grid with space-generated solar power. In
Stage 3, large ultralight GEO reflectors will beam light to the direct-conversion collectors, and multiply the power through the
grid. The need to put expensive, heavy solar arrays in high orbit is avoided, along with the objections to atmospheric transmission
of visible light. The system would gain significantly from the development of low-mass, high-efficiency conversion equipment
for direct conversion of broadband solar energy to beamed microwaves.
Keywords: Space solar power. Microwave beaming.
INTRODUCTION
Space Solar Power (SSP) remains an unfulfilled dream for three
reasons. They are the immense launch cost (Schwartz 2001, Smith
2003); lack of an evolutionary approach (cost to first power
>$300B; McSpadden 2002); and community opinion that local
solar collection is sufficient for present needs. Given their high
retail costs and unsteady nature, terrestrial solar-electric and wind
power sources still remain secondary and subsidized. It discusses
how to develop an evolutionary approach where revenue
generation starts early. This paper serves to explain the concept
and explore the major issues.
The key feature is to use the potential of the space-based
infrastructure to boost terrestrial “green” energy production and
thus benefit from the concerns about global warming and energy
shortage. In this first paper on the concept, the scope of the project,
possible benefits and the obstacles to success are considered. It is seen that the inefficiency of conversion to and
from microwave poses the largest obstacle, and prevents favorable comparison with terrestrial high-voltage
transmission lines. However, competitive revenue generation can come from the nonlinearity of cost with demand at
various places on earth. Point delivery to small portable, mobile receivers during times of emergencies. The benefits
to ‘green’ energy generation make the concept attractive for public support as a strategic asset. This also sets a
market context for concepts to convert solar power directly to beamed energy – a prospect with many applications.
FIGURE 1: Space Power Grid satellite
receving and redistributing beamed power.
Previous Work
Earlier concepts used GEO PhotoVoltaic array satellites and kilometer-sized terrestrial microwave collectors. A
1995 “Fresh Look” study (Mankins 1997) proposed more radical concepts, but the basic issue of launching the
conversion equipment (1kg/kw) remained. Some have argued that pooled SSP demand for over 5000 launches to
GEO would cause a collapse of launch costs. Kellum (2004) looked at scenarios enabled by space elevators, but
even at $100/kg launch cost, $0.12/kwH energy price and $100/acre receiver land cost, SSP was competitive only by
using direct-conversion optical rectennae (which are still to become practical). Criswell (1998) has argued for
locating collectors and conversion equipment on the Moon, pointing out that a transmitter with very large synthetic
aperture can be built on the Moon for less than the cost of one solar power satellite. Ignatiev (2000) has shown how
to vacuum-deposit solar cells using lunar “rovers”, and extend this to mass-scale lunar power generation. Clearly, at
today’s pace, lunar sun-power is also a distant prospect.
Concepts have been proposed for low-earth-orbit (LEO) satellites beaming power to earth, to alleviate the launch
cost problem. This raises the difficulty of short, intermittent transmission and active tracking; however technological
advances have made these acceptable. The shorter transmission distance (1,200km to LEO vs. 36,000 km to GEO)
greatly reduces antenna size and mass. Soubel (2004) and JAXA(2004) describe a Japanese project to construct a
50KW demonstration satellite in LEO followed by a 10MW satellite in a 1,100km-high orbit, giving 200 seconds of
power to ground receivers during each pass, with an orbit period of roughly 90 minutes, with retro-directive power
beaming to enable tracking. This is proposed for retail power beaming to devices such as cellphones. Hoffert (2004)
proposed an evolutionary technical approach to space solar power demonstration, stepping up through terrestrial
point-to-point beaming, intermittent beaming from LEO to consumers in developing nations combined with storage
devices, and beaming/reception demonstrations to high orbits using large facilities such as the Arrecibo radio
telescope. The evolution here was however limited to government-funded confidence-building for full-scale SSP
rather than early revenue, so that the basic SSP problem of the huge cost-to-first-power remained.
Escalating hydrocarbon costs promise to shatter community complacency and thus alleviate the third obstacle to
SSP. The cost of emitting carbon is also a new factor. The net economic costs to the US from either signing, or
trying to compete in a world that has signed the Kyoto Protocol, have been estimated at $160 to $260 per ton of
atmospheric CO2 released (GlobalWarming, 2000). The US DOE provides one “Green Tag” per MW (Adler 2005)
to producers of clean alternative energy, tradeable on the market like the “carbon credits” of the Kyoto Protocol.
House Resolution 759 proposes stringent CO2 reductions, with further “green” credits (Gilchrist 2005) citing
penalties averaging over $15 per ton of CO2. Northwestern states already offer various incentives for solar energy.
Total worldwide electricity generation from 49,000 power plants is over 2,812 GW. North-American plants (4,144)
average 265MW, with 774 plants producing an average 310 MW of “green” energy. Plants located elsewhere
average 38MW. “Green” plants using solar and wind energy suffer from large cyclic and random fluctuations. Realtime power smoothing can turn these plants into reliable “firm sources” (Fairey 2003), greatly increasing their value
and revenue potential. Wind and solar plants are best located far away from primary consumption areas, if
transmission line costs are removed from the equation. Examples are South Dakota, called the “Saudi Arabia of
Wind Energy”, and Southern Nevada and the High Desert of New Mexico for sunlight. For usual power plant
choices, current transmission line costs are estimated at roughly $0.02 per KWH in the U.S. Transmission loss is
estimated at only 7.2% in the U.S., but is as high as 50% in many countries with decrepit infrastructure and lower
line voltages. Power costs to the consumer vary widely around the world. In the U.S., base load costs are around
$0.06 to $0.08, but in the U.K., with a substantial fraction of power coming from wind and other “green” sources,
the cost is as high as $0.22 per KWH, with other nations somewhere in between. In the U.S., cost during peak
demand times increases sharply, for instance during the Californian experience of a few years ago. Thus the
alternative-energy literature cites the extreme difficulty of setting up plants that can compete in the U.S. With our
concept, U.S.- based sources could compete quite well in markets around the world.
Project Scope
The total solar energy falling on the projected area of the planet is roughly 1.28*108 GW. Thus, making a 100%
improvement in global energy production using renewable energy sources implies a system that captures roughly
0.002% of the total solar energy falling on earth. Very large collectors can be placed in space at GEO or beyond,
without obstructing sunlight coming to earth, so the long-term potential for SSP is essentially limited only by the
need to resolve concerns about adding heat to Earth. Thus the overall scope of the project is limited only by eventual
concerns about handling excess heat added to Earth – when the system gets so large that solar power outside Earth’s
capture area is directed in.
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APPROACH
We propose a space-based power transmission grid to exploit the above opportunities, initially with no space-based
generation. The issues here are technical feasibility, end-to-end efficiency and competitiveness of cost when viewed
in terms of overall economic impact. In the first stage, the aim is to create a space-based grid to handle power
transactions. The competition is earth-based transmission for a new set of “green” plants all over the world, enabled
in locations remote from industrial and population centers. Added impetus comes from the demonstration of national
preparedness to cope with natural or other disasters. The December 2004 tsunami in Asia and the 2005 U.S.
hurricane season demonstrated the vulnerability of power grids, and the opportunity for using space-based power
delivery to disaster areas. This opens up an opportunity to bootstrap SSP using a national / international strategic
imperative. In a second stage, to occur 10 to 15 years later, some space-generated power would be added to the
satellites of the grid, with the delivery infrastructure already in place and generating revenue. It is reasonable to
project that technology for direct conversion of sunlight to microwave energy will be advanced enough by then to
allow much greater power per unit deployed cost than present-day solar cells. At any rate, this equipment will be in
low earth orbit, not GEO. In a final stage, very large thin-film reflectors or Fresnel-lens structures will be placed in
GEO to focus sunlight directly onto the converters in LEO, thus delivering large amounts of power to a scaleable
infrastructure.
This scheme offers a solution to the economic dilemma of local generation vs. distribution from generating hubs, by
beaming energy into space from ideal collection locations, and redirecting it to earth-based microwave collectors for
local distribution. It also minimizes the space-power launch cost issue by doing all collection and conversion on
earth, and only reflection/ distribution in space initially.
The key enablers will be advances in ultra thin microwave receivers, “smart” digital adaptive antennae, micro
thrusters, swarm guidance and navigation, power beaming and high-intensity waveguides; however in each of these
areas, what we propose is no more advanced than what is being considered for other SSP approaches. The scope of
the initial system is small. The initial power grid, sized to a $5B investment, leaves plenty of room for later
expansion of the system to SSP. These stages in the system are illustrated in Figure 2.
INITIAL SYSTEM PARAMETERS
Space Power Grid Phase
An initial concept is to have 36 satellites orbiting 1,215 km above earth (determined to allow line-of-sight
transmission to a satellite 45 degrees away without atmospheric losses) and passing microwave beams between
earth-based locations in a real-time energy trade. Each craft can handle (receive and distribute) upto 200MW at 140
GHz. Each is assumed to be able to transact power with 4 other satellites and up to 100 ground stations. The craft
are sized to recover system deployment cost in 20 years from savings in costs of ground transmission, based on
current GPS satellite costs (as explained below). The satellite number will rise to 72 as locations far from the
equator join the system. Beaming losses to and from LEO are density-equivalent to those for 22 km of transit
through sea-level atmosphere. Hence the business case applies to new plants located over 22 km from their major
distribution hubs. Satellites in equatorial / tropical orbits will be capable of receiving more power from the ground,
and transmitting more power to outlying satellites. Satellites in inclined orbits will distribute more than they collect
from ground.
Ground stations will be located at power plants to generate and beam microwaves, at ideal solar / wind collector
locations – dry high-desert locations where land is cheap and sunlight or wind abundant, and mountainous / coastal
ridge regions with high winds. The US Southwest, South Dakota, Hawaiian Islands, North African, Gobi, Thar and
Australian deserts and Greenland are examples envisaged. Receiving stations on the ground can be located almost
anywhere. There is no need to co-locate receiving stations with generator stations. Since receivers will be much
smaller in diameter than those envisaged for GEO-located SSP systems, a given utility company can place its
generators at optimal generator locations, and its receivers to best distribute power to end-user customers.
The tradeoff between the choice of frequency (preferably below 10GHz to minimze atmospheric losses) and antenna
size for a desired 99.93% transmission efficiency of Gaussian beams, (JAXA2004) poses a dilemma. The limiting
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transmission is between satellites (2,400km). We choose 150m diameter antennae on the satellites as a reasonable
choice for an LEO satellite to be launched on a single launcher. This means that the frequency must be at least
100GHz. We choose 140GHz in order to use calculations from the literature (Parkin 2003). In this regime,
absorption by water molecules is significant but not prohibitive, and transmission is generally above 90% (ARO
2005). This choice will encourage location of transmitting stations in high deserts and mountainous regions, but will
pose inefficiencies for delivery to places with dense cloud cover. A partial solution is to deliver power to places
without cloud cover and use existing ground-based transmission lines for the remaining distance; however, this is no
solution where lines are down and cloud cover persists, as in a prolonged snowstorm. Unlike the GEO SSP concepts,
the LEO system can start generating revenue with roughly 20 ground-based generating stations around the equator,
and 20 LEO satellites in orbits within 10 degrees of the equator. Advanced technology can be progressively
incorporated into routine replacements and additions. As the earth-based transmitter/receiver/ distribution
infrastructure proliferates, market size will increase.
Direct-Conversion Augmented SPG
We project that Direct Solar Conversion to microwave beams will become feasible with 50% efficiency and reduced
mass by 2035. To replace current global production with solar energy at 50% efficiency, 5600 sq.km of solar
collector area in space (where solar intensity is 1GW/sq.km) is required. The breakthrough needed for this is nanofabrication technology, and the same will also permit better conversion and beaming efficiency. The SPG satellites
will then be replaced with 2,000kg–class Direct Conversion Augmented-SPG (DCA-SPG) satellites, with a 1km
diameter sun-tracking ultra light collector and converter on each adding 0.5 GW to the grid. As the number of
satellites increases, mean transmitting distances between satellites comes down, and hence the system efficiency
could be increased by going to lower frequencies with lower atmospheric losses for the same receiver size.
Full Space Solar Power Phase
GEO sun-sats launched in the 2040s will each have 100 sq.km ultra light collector/ reflectors that simply focus
sunlight onto the 1sq.km collectors of the DCA-SPG. Each is expected to add 50GW to the grid at 50% efficiency.
Thus the system of 72 LEO satellites and 72 GEO ultralight mirrors, with a 70% transmission efficiency, will
generate 90% of today’s global energy production. It is noted here that deploying large ultra-thin collectors with
high-intensity solar cell arrays is an alternative to any Direct Conversion technology, alleviating technological risk.
Since the system is in LEO, the launch cost is far below that of launching solar panels to GEO.
TECHNOLOGY ISSUES
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Low distributed earth orbits enable much smaller receivers on the satellites. They also permit the participation of
sites at extreme latitudes with minimal atmospheric transmission penalties. The sizing of the transmitters in space is
driven by the need to focus beams onto other satellite-based receivers – a 2,400 km line of sight in orbit, that will
come down as the number of satellites increases. One substantial advance is in “Smart” antenna arrays (Cooper
2003) that use digital computing to focus on thousands of individual moving cell-phone customers in real-time. One
approach is to have 2km-long linear antenna arrays with up to 32,000 elements. Transmitting arrays on earth stations
would be of similar dimensions, but much heavier and designed to transmit much greater power levels. Waveguides
to distribute incoming power are well-developed, and the power levels we propose for the initial system are easily
handled by present designs for microwave guides. Atmospheric absorption is not a serious issue , and microwave
conversion and beam transmission at 2.45GHz are well-understood technology (Brown 1984). At 140 GHz,
atmospheric transmission loss is on the order of 10%, for stations with dry climates, but transmission through clouds
poses more serious problems. Transmission at higher frequencies offers small size and mass for the same efficiency.
The tradeoff between system mass costs and the cost of atmospheric absorption and unreliability due to weather
(alleviated by having multiple earth station choices separated by several kilometers) is not properly understood,
since much of the high-frequency data comes from astronomical observatories until now. It is not known whether a
combination of high frequencies for space-space transmission (distances of 2,000 km) and low frequencies for earthspace transmission (1,200 km) may hold advantages, with efficient conversion between frequencies on satellites.
Today, such conversion is inefficient. A breakthrough is needed in this area. Likewise, the DC-microwave
conversion is the limiting step in the entire process, with efficiencies limited to about 70%.
1.Microwave converters and beaming equipment installed.
2. Thirty-six 200-MW SPG satellites launched.
3. SPG in operation.
4. Direct converter-augmented satellites: DCA-SPG
5. SSP collector beams sunlight to SPG: Full Space Solar Power
FIGURE 2: Evolutionary path to full Space Solar Power.
In our system, the primary power generation
can come from any source, though there is not
much point in using another technology that
generates CO2 or nuclear waste. Current
“green” options for utility-scale power
generation are direct solar thermal conversion,
and wind turbines. Present direct solar thermal
farms heat an oil-salt mixture in arrays of long
pipes placed along the focal lines of parabolic
reflectors placed in east-west trenches to track
the sun. The heat is transferred to run a steam
turbine that generates electricity at 5 to 9 cents
per KWH. Installation costs are low. With
SPG, solar and wind power generation facilities
can be located where they are most productive,
and their power sold wherever they fetch the
best prices, by beaming them through reflector
satellites. Uneven generation rates are
compensated by power brokering through SPG.
Heat rejection
At the 200MW peak power level involved in the transmission through a satellite, even a 1% loss means 2 MW of
heat dissipation, aggravated by the small size and mass of these satellites compared to SSP designs. However, with
large thin receiver arrays attached by booms to the satellite, radiation transfer from the shadow side can suffice for
thermal management. The intermittent duty cycle, as opposed to the steady heating of SSP satellites in GEO, helps
in this process, but innovative means of recovering part of the power and efficiently rejecting the rest are needed.
Thus, increasing the efficiency of the satellite throughput is of utmost importance. However, waveguides today have
demonstrated near-100% throughput.
Direct Conversion
One key to advancing towards Space Solar Power is the technology of Direct Conversion using optical rectennae
that convert sunlight to DC, and possibly extensions of this where sunlight is directly converted to microwave. This
promises a large increase in efficiency (over 80% from DC as opposed to the present theoretical 40% of solar-cell
systems), and a large payoff from decreased converter mass from the present 1kg per kilowatt (Brown 1984). More
exciting is the prospect of converting sunlight to microwaves directly at 85% efficiency to microwaves. The
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technical barriers appear to be in nano-fabrication of antennae that can tune effectively to much of the solar
spectrum. Rapid progress is expected in this field in the coming decade (Berland 2003).
Power Handling
Outside the beam weapon community, it is hard to find experience on the best that can be done in microwave
antennae and waveguides in transmitting 200 MW of power.
LEO Interference
Microwave beams pose no direct thermal threat to satellites; however, they may interfere with satellite electronics
and with communications. The addition of 36 satellites will not seriously clutter LEO; however, collision with space
debris is a serious risk. We have no immediate solution for this, except that the orbits are higher than most LEO 3rdstage separation junk. The impact of Earth’s magnetic field on transmissions must be considered.
Top-Level Breakeven Cost Estimation
Several of the issues listed below are beyond the scope of this paper but may determine market feasibility.
1. Average power generation cost around the world a couple of years ago was between $0.04 and $0.08 per KWH.
However, this was dominated by the low cost of hydrocarbon fuels. Hydrocarbon costs have tripled.
Transmission costs are typically cited at 0.02 per KWH in the US, but may be substantially higher elsewhere
due to much higher loss percentages and land costs. Both of these costs are on the rise as “Peak Oil” appears to
have been reached, and land values and environmental impact costs of power lines go up.
2. Interest rates on long-term debt fluctuate considerably.
3. The cost of carbon-based energy sources may be expected to rise further as environmental concerns impose
strict limits and penalties. The global need to switch to non-hydrocarbon sources justifies some level of public
(government) expenditure on infrastructure to enable ‘green’ power.
4. Several nations would trade their ‘green credits’ and buy power from nations that sell power generated through
whatever means – if they could get that power.
5. At present, high efficiency is not a driving consideration in microwave power beaming, since antenna size, mass
and cost are more important for transmission at low power values. A value for achievable conversion efficiency
at high power level is hard to find. Most experts appear to assume a 70% practical limit using present-day
technology, for open-system conversion, with 30% dissipated as heat at the conversion point. In a power plant
context, it may be assumed that a good portion of this can be recovered – we assume that enough can be
recovered to compensate for the 20% loss of transmission through the atmosphere and bring the loss rate to the
7 to 8% now lost in the transmission process. We expect that in future, dedicated plants to generate microwave
beams can be developed to minimize the conversion loss from raw sources. Thus, the transmission approach
that we propose only transmits 70% of the power generated.
The above items make it difficult to assign a value per KWH for power transacted through SPG at this stage of the
concept. For simplicity, we assume that the space-based power transmission stage has a value of $0.02 per KWH
transacted. To break even over the 20-year replacement period of SPG-stage satellites, the initial 36 satellites must
have at least a capacity of 136MW each. This assumes that all power beaming from earth is on the sunny side (only
18 sats in view) and that each ground station has a choice of 4 satellites for its beams at any time. A market size of
5,500 GWH per year of power transmission must be achieved. This appears quite reachable for an initial
consortium agreement. We project SPG launch cost of $6600 per kg to a 1,215km high orbit. Assuming satellite
mass of 1,500kg, and cost per satellite of $22M, there is enough left from an initial $1.7B bond issue to fund one
replacement satellite per year.
The most recent GPS satellite launches provide cost guidance on a government-developed dual-use system. The cost
per 2,036kg satellite is $44M, and the launch costs $50M (or $24,557 per kg). The orbit is over 1,0000km high.
The GPS model is inadequate as a reference, because of the huge user base and primary military application.
End-to-end efficiency
At present, the end-to-end efficiency of this process alone does not compare favorably with earth-based transmission
of energy, in existing markets. With 70% at conversion, and 10% for each atmospheric pass, even with essentially
100% waveguides and in-space transmission, the end-to-end efficiency is limited to roughly 50%, compared to
about 90% for transmission over high-voltage lines. However, this masks the value of the approach in opening up
6
worldwide markets, smoothing power fluctuations, avoiding loss of the “excess” power of ‘green’ energy plants,
and enabling power plants in remote areas and connecting them to new development in other remote areas. A more
detailed examination of the economics and policy aspects of the concept must wait until a later paper, where we
expect to show how the inclusion of these large-system aspects, typically neglected in engineering concept
development, make all the difference here.
TABLE 2: Issues Comparison with Conventional SSP and Terrestrial Solar Approaches
Energy
Production
Launch cost
Space Mass
Cost Items to
First Power
Duty cycle
Assembly
Global
coverage
Transmission
losses
Transmission
line
Breakeven
SSP
Primary solar generation
>$13,200/kg to GEO
>1kg/kw
Sats + grnd rec’ers
24hr w/ reflectors
LEO assembly, boost to GEO
inclined
GEO-earth: 15% > 24,000 km
space + 1 atmosphere
0
>>$1/KWH
SPG
Exchange; new terrestrial plants,
Augmented SPG, then Full SSP
$6,600/kg to 1215km alt. orbit
<0.01kg/kw for SPG phase
Space system + grnd x’mission &
rec’ng + control.
24 hr – with multiple sources
Pre-assembled – deploy in LEO.
direct
Terrestrial Solar
Constrained by duty
cycle, & location
N/A.
N/A
Gnd system+ line +
land costs.
6hr/day; weather
Earth construction
no
Conversion loss: 25%; Earth-LEOearth: <20% Average <5,000km
space + 2 atm. traverses.
0
7%
~ 1.8 x the lowest terrestrial
generation/transmission costs.
~ 2x lowest terrestrial
generation/transmission
costs in the U.S.
$2c/KWh
CONCLUSIONS
By carefully integrating environmental and energy policy issues, and rethinking the concept of SSP, we show that a
viable, realistic, socially and politically acceptable technical path can be laid to reach the dream of Space Solar
Power. While a simplistic calculation of end-to-end efficiency shows beamed power transmission to be far inferior
to transmission via high voltage lines, a space-based power grid opens up various markets and opportunities that are
otherwise closed. The single most important point of the present concept is that it provides the long-sought
Evolutionary Path towards Space Solar Power. The inverted thinking of SPG, where we initially beam power into
space rather than from it, is the key. Initial system size and scope are kept small to enable deployment and revenue
generation, giving time for market forces to identify the opportunities. The production/deployment cost of a 36satellite system is covered to at least 50% from transmission line costs of the power transacted, with substantial
added benefits from carbon cost savings. Advancement to Stage 2 will require improved microwave power handling
(waveguide) technology, and in nanoscale fabrication for direct conversion to microwaves.
ACKNOWLEDGMENTS
Part of this work (microwave beaming and direct conversion issues) was conducted in the course of studies related
to a grant from the NASA Institute of Advanced Concepts. SW and NK acknowledge the support from the John D.
and Catherine T. MacArthur Foundation through the Sam Nunn Security Program in the School of International
Affairs, Georgia Institute of Technology to study the broader policy issues.
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